Costello started 25 games during his time with Stanford, and will now have the perfect coach to develop him in Leach, who led the Washington State Cougars as the nation's top passing offense in four of his six seasons there. Leach got a close up look at Costello in 2018, as the QB threw for 323 yards and four touchdowns in Stanford's win that Saturday.
The now former Cardinal posted a meaningful thank you message to Stanford on Instagram, stating "Dreams do become reality," while also adding "excited for what is next." He did earn second-team All Pac-12 honors in 2018, throwing for 3,540 yards in 13 games, which is the second-highest total in school history. He also added 29 touchdowns and completed 65.1 percent of his passes, but that production went down the following season. 2019 saw him deal with numerous injuries, as he only appeared in five games and finished with 1,038 yards passing, six touchdowns, and three interceptions. Before the season, it was expected that he would enter the NFL Draft this April, but injuries changed everything, and even led to a change of scenery for the 2020 campaign.